When your computer first starts up, at the time where it shows a big splash screen for the PC manufacturer or the information about what memory and disks its found, it's doing something called a Power-On Self Test, or POST, for short. This is just checking to make sure all the components are working properly. If the power-on self-test needs to tell you something, about the only thing it can do is beep at you. This is because, at the time the POST does its job, it doesn't know if there's a working screen, or keyboard, or sound-card (the beep is a separate speaker, not the proper sound-card). Each manufacturer uses a different system of beeps, depending on what the message is. What you'll need is the name of the motherboard manufacturer - this will be AMI or Nvidia or Phoenix, or some company like that, not necessarily the people who sold you the computer. You can find this out from Control Panel -> System and then look for a list of the hardware you have. You'll need to look up the motherboard manufacturer, and do a web search for their name, for 'post beep codes' and see what comes up. It might be anything from a loose connector to a dodgy power supply. Or, you could take it into a PC repair shop and ask them to have a look at it.

perhaps there is an internal fire and the noise that you can here is the computer's version of a smoke alarm.